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Full text of "Alaskan glacier studies of the National Geographic Society in the Yakutat Bay, Prince William Sound and lower Copper River regions"

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170                                 ALASKAN GLACIER STUDIES
,> shaking.   The Marvine Glacier, on the other hand, seems to be much larger than either
of these, and certainly is if the Malaspina extension of Marvine Glacier is included. We do not know what the conditions are in its upper portion, and it is possible that the advance is the result of a thrust from some of the shorter tributaries; or it may be true that this glacier is a steep one, though broad and flat where it emerges from its mountain valley, expanding far beyond the mountains because of large supply grounds at no great distance back in the mountains. The best map of the region that we have indicates that the latter is true, and that the Marvine Glacier is little if any longer than the Variegated Glacier and much shorter than the Lucia Glacier. If this is not true, it is certainly noteworthy that this glacier should have advanced as early as the Atrevida, and three years before the Lucia did. If we could depend absolutely on the map the question would be easily settled.
By the advance of these four glaciers the ice surface was in each case profoundly broken, being transformed from a surface over which one could easily travel in 1905 to one which was utterly impassable in 1906. In at least two cases, the Atrevida and Variegated Glaciers, there was a lateral spreading of the ice within the mountain valley, closing up marginal valleys which had previously existed. In these two cases, and probably in the others also, there was a pronounced thickening of the glacier at and beyond the mountain valley mouth. Three of the glaciers—the Marvine, Atrevida and Variegated —terminate in expanded piedmont bulbs, and in each of them the breaking of the ice surface extended into these ice bulbs, in the Marvine reaching to the very outer edge, but in the two others failing to affect the outermost parts of the stagnant ice. From this we infer that the thrust in the Marvine Glacier was the most pronounced of all and this inference is supported by the further fact that the extent of the area of broken ice was far greater in the Marvine than in the other glaciers. The area of broken ice visible in the Marvine Glacier, and its continuation in the eastern lobe of Malaspina Glacier, was fully 15 miles in length and from 8 to 10 miles wide in the broadest part; in Atrevida and Variegated Glaciers the broken ice occupied an area whose length was between 5 and 7 miles, with a breadth of % or 8 miles in the broadest part. All four of the glaciers advanced along their fronts andmargins, the greatest advance being in the Haenke Glacier, the only one whose terminus was confined between mountain walls. Here the front of the glacier was pushed forward nearly a mile and it was transformed from non-tidal to tidal condition by the thrust. The forward motion in the other three glaciers was partly dissipated ip. breaking the piedmont area and partly in thickening the ice in it; but some of the thrust was effective in pushing the margins of the piedmont areas outward. This was clearly shown in Variegated Glacier by the overriding of a granite gorge; in the others by the advance of the margin into the fringing forest.
Thus, any theory which will account for the transformation of these glaciers, must explain not only change from stagnant condition to activity, and from smooth to broken surface, but also actual forward movement and pronounced thickening of the glaciers. The theory must also account for the suddenness of the change. At least three of the glaciers, Atrevida, Haenke, and Variegated, and probably also a fourth (the Marvine) passed through the complete cycle in approximately one year. In the last of August, 1905, they showed almost no evidence of change, though in the Atrevida and Marvine Glaciers there were observed phenomena in 1905 that were later interpreted as the beginning of the advance; in June, 1906, all four glaciers were utterly transformed. The